That was a mistake, I meant to write "Casfaidh", but "Casfar" would do too.

lena chéile and le chéile - I'm not sure what the difference is. I think both are interchangable.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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This is great. Makes me really want to study again!

So which version would fit best for the song text?

Dá dtitfinn chun deiridh
Má thitim chun deiridh
Má thitim siar

At this point, it's more a matter of which you like best (I added a fourth choice to your three), choosing between chun deiridh meaning "behind" (or, more literally, "to the rear") and siar meaning "back". Whether to use dá or má is also largely a matter of choice, at least in this context. It isn't always true, but to some extent dá is used for something which is unlikely to happen or not necessarily expected to happen, while má is more of a generic "if", without judging the likelihood.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Out of the four options in Caoimhín's post above, I think I'd prefer Má thitim siar - for one thing it would save a bit of time under the needle
You could consider having it done in the Irish script (note the dot instead of the 'h'):
Má ṫitim siar
In caps:
MÁ ṪITIM SIAR

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Thank you so much for your help. Sorry for not replying sooner, was travelling and couldn't access the site from my mobile.